Intensity attenuation law in the eastern flank of Mt. Etna

The eastern sector of Etna volcano is crossed by faults trending NNW-SSE, locally called Timpe. Their significant activity is marked by both the geomorphological evidences and the seismicity. Most of the sources of the shocks taken into account are associated to such tectonic structures. The earthquakes have a moderate magnitude (M<4.5) and very shallow depth (h<3 km). As a consequence of these peculiar features, the damage occurring, that sometimes is quite heavy, affects very small areas. Moreover, these characteristics imply that we cannot adopt the usual attenuation laws evaluated for regional size earthquakes. Aim of present paper is to evaluate the peculiar attenuation law of the macroseismic intensity in the eastern flank of Mt. Etna. The macroseismic observations of local seismic events, reported in the database of Etnean earthquakes, were used as input data, taking into account 58 earthquakes having epicentral intensity Io > 5-6 and at least 11 site observations. The analysed data were tested performing linear regression analysis as well as exponential and logarithmic ones. The results pointed out that the best fit is achieved by adopting an exponential relationship like I = a e -bx . All the obtained relationships put however into evidence a strong dependence of the attenuation on both the maximum intensity and the hypocentral depth of the events.